weight vs colour

You are absolutely right that the shear does not actually change the average color of the glyph in its box, even if you would subjectively evaluate the width of the slanted line as thinner than the straight line.

Proper implementation of font blending gets gamma right, even if linux software that does it correctly is very scarce -- in fact nonexistent would be more accurate. As an aside, I was able to get sRGB surface support in the 0.27.2 release of pixman, though, so maybe if I make more noise about this people start to use sRGB surfaces when blending text...